The option that would increase competition between the two species of fish is (c) an increase in the number of food plants in the pond.
Competition between two species occurs when both species use the same resources, such as food, water, or habitat. In this case, the two species of fish eat the same kinds of plants and are eaten by the same water birds.
If the number of food plants in the pond were to increase, this would increase the number of resources available to the fish. As a result, the two species would have to compete for the same food resources, and the competition between them would become more intense.
The other options are not likely to increase competition between the two species of fish in this scenario. An increase in average habitat temperature (a) may affect the behavior and physiology of the fish, but it is unlikely to directly impact competition between them. An increase in the population of water birds (b) would likely increase predation pressure on the fish, but it would not directly affect their competition for resources. Finally, a decrease in the water level in the pond (d) may limit the available habitat for the fish, but it would not directly increase their competition for resources.
~~~Harsha~~~